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Embryology and cytogenetics of apomictic triploid Eupatorium squalidum DC. (compositae)

James R. Coleman; Marina A. Coleman

Departamento de Botânica, Instituto de Biociências, Letras e Ciências Exatas, UNESP, 15100 Sâo José do Rio Preto, SP, Brasil. Send correspondence to J.R.C.

ABSTRACT

Embryological studies demonstrate Eupatorium squalidum to have Antennaria type diplospory with pronounced precocious embryony. Endosperm development is autonomous, and the central nucleus divides to initiate the endosperm only after the embryo is well developed. It is estimated that about 10%; of the florets reach anthesis with the egg still present. Presumably, these florets would be available for sexual reproduction. Each of the six plants in which microsporogenesis was studied demonstrated the presence of univalents, bivalents and trivalents during meiotic prophase. Failure of first division frequently resulted in the formation of restitution nuclei and, following second division, the formation of dyads of unreduced microspores. A correlation is noted between univalent formation and dyad production. Chromosome counts of the progenies of 25 mother plants revealed that 32 (5.19%;) of the 617 progenies had chromosome numbers variant with those of their mother plant. Thirty-one of the variant progenies had 60 chromosomes and one had 48. It is considered probable that the variant progeny resulted from sexual reproduction involving the fertilization of an unreduced egg by sperm with 30 or 18 chromosomes. The formation of variant progeny with predominantly 60 chromosomes suggests the unreduced pollen to be functional. Plants with 60 chromosomes were not encountered established in nature, which suggests that they do not survive to maturity or, if they survive, are not recognized as E. squalidum. Three aneuploid plants with 69 chromosomes were encountered established in nature. A karyotypic study revealed the chromosome complement to be formed of a basic set of ten chromosomes repeated three times, which indicates that the material studied is of autotriploid origin.

Keywords: embryology; cytogenetics; apomictic; triploid; Eupatorium squalidum.

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